1. Field of the Invention
This invention relates to an iris diaphragm device for use in a camera and, more particularly, to an iris diaphragm device which can be compactly mounted within cameras of various types such as a still picture, a cinecamera and a video camera and applied to an automatic exposure control system for automatically adjusting the size of an iris aperture in the optical system to determine appropriate exposure in accordance with the quantity of light of a subject.
2. Description of the Prior Art
An iris diaphragm device incorporated in optical systems in cameras of various types such as a still picture camera, a cinecamera and a video camera is generally composed of an iris diaphragm element having a plurality of iris diaphragm leaves which can be swung about their pivots and a drive means for opening or closing the iris diaphragm leaves to determine appropriate exposure in accordance with the brightness of a subject detected by means of a photoelectric device such as a charge coupled device (CCD). Such iris diaphragm leaves in the diaphragm device for a camera are rotatably supported by pivots so that the diaphragm leaves can rotate around the pivots to be opened or closed relative to the optical axis of the optical system, thereby to form an aperture through which the light rays pass. The diaphragm device of the construction discribed above is generally held in position by a ring-shaped or cylindrical base member and fixedly incorporated inside a camera body or a lens barrel. On the other hand, the drive element for causing the aforementioned diaphragm leaves to be opened or closed is usually composed, similarly to an electric motor, of an electromagnetic coil and one or more magnets for applying a magnetic field to the eleclromagnetic coil. This drive element has been disposed outside the aforementioned base member in such a state that the motive power is transmitted to the aforementioned diaphragm device through the medium of a leaf operating means so as to open or close the diaphragm leaves. Therefore, the drive to be incorporated into the optical system inevitably protrudes in the shape of a ring from the outer surface of the lens barrel. The protrusion of the drive element from the lens barrel is both unsightly and unhandy.
Recently, a need has been felt for a small and light parts including the diaphragm device in a video camera or the like. Specifically, in order to provide a camera of good appearance from the standpoint of design, the outer diameter of the diaphragm device are desired to be close to that of the lens barrel in the camera.
Mechanisms intended to reduce the outer diameter of the diaphragm device by mounting the drive element for effecting opening and closing of the iris diaphragm leaves within a base member for fixing the iris diaphragm leaves have been proposed in Japanese Patent Application Public Disclosure Nos. SHO 54(1979)-99424 and SHO 48(1973)-15524, for example.
The former mechanism consists of iris diaphragm leaves distributed at equi-distance about an aperture and magnetic rotary members of small size as many as the diaphragm leaves, each disposed outside the respective diaphragm leaves, so as to cause opening or closing motion of the diaphragm leaves by angularly rotating the magnetic rotary members in order. The megnetic rotary member in this mechanism has a structure similar to an electric motor, that is to say, it is constructed merely by compactly containing an electromagnetic coil and magnet rotors into a small-sized cylindrical yoke. Because of the construction described above, this diaphragm mechanism is complicated in structure and exhibits inferior driving power for operating the diaphragm leaves. There is a possibility that this mechanism will be incapable of providing required opening or closing motion of the diaphragm leaves.
The latter diaphragm mechanism comprises a plurality of magnets circularly disposed in a row, a circular guide rail arranged in parallel to and above the row of the magnets, and moving coil members mounted slidably on circular guide rail. The coil members are linearly moved along the guide rail by applying an electric current to the moving coil members in accordance with the brightness of a subject, thereby to open or close the diaphragm leaves.
Neither of the diaphragm mechanisms described above assures stability of the synchronous motion of the moving elements. Thus, these mechanisms necessitate a means for finely compensating the difference in motion among the moving elements and cannot be used practically.
To overcome this fault found with the conventional diaphragm mechanisms, there has been proposed a practical diaphragm device of small size which assures reliable motion of the diaphragm leaves (Japanese Patent Application Public Disclosure No. SHO 58(1983)-80627). The proposed diaphragm device comprises a pair of arcuate magnets disposed across an iris aperture and a circular moving coil concentrically arranged relative to the magnets and rotatably supported at the both sides thereof by means of pivots, so that the moving coil is rotated around the pivots by supplying to the moving coil an electric current corresponding to the brightness of the subject, thereby to cause a plurality of diaphragm leaves to be simultaneously opened or closed. This diaphragm device has an outstanding advantage that it can effectively open or close the diaphragm leaves, whereas the pivots extending outwardly from the sides of the moving coil prevent miniaturization of the diaphragm device because the pivots somewhat project outward. Furthermore, the privots are integrally formed on pivot fixing seats and fixed to the sides of the moving coil by the medium of the pivot fitting seats by use of adhesives and so on. However, if the portions to which the pivot fixing seats with the pivots are fixedly adhered has large curvature, it is difficult to completely fix the pivot fixing seats to the circular moving coil.